Sunday, July 29, 2012

genomic study of african hunter-gatherers elucidates human variation and ancient interbreeding

sciencedaily | Human diversity in Africa is greater than any place else on Earth. Differing food sources, geographies, diseases and climates offered many targets for natural selection to exert powerful forces on Africans to change and adapt to their local environments. The individuals who adapted best were the most likely to reproduce and pass on their genomes to the generations who followed.

That history of inheritance is written in the DNA of modern Africans, but it takes some investigative work to interpret. In a report to be featured on the cover of the Aug. 3 issue of the journal Cell, University of Pennsylvania geneticists and their colleagues analyze the fully sequenced genomes of 15 Africans belonging to three different hunter-gatherer groups and decipher some of what these genetic codes have to say about human diversity and evolution.

The study, led by Sarah Tishkoff, a Penn Integrates Knowledge Professor with appointments in the School of Arts and Sciences' biology department and the Perelman School of Medicine's genetics department, tells several stories.

It identifies several million previously unknown genetic mutations in humans. It finds evidence that the direct ancestors of modern humans may have interbred with members of an unknown ancestral group of hominins. It suggests that different groups evolved distinctly in order to reap nutrition from local foods and defend against infectious disease. And it identifies new candidate genes that likely play a major role in making Pygmies short in stature.

"Our analysis sheds light on human evolution, because the individuals we sampled are descended from groups that may have been ancestral to all other modern humans," Tishkoff said. "A message we're seeing is that even though all the individuals we sampled are hunter-gatherers, natural selection has acted differently in these different groups."

Joining Tishkoff in the work from Penn was first author Joseph Lachance as well as Clara Elbers, Bart Ferwerda and Timothy Rebbeck. Their collaborators include Benjamin Vernot, Wenqing Fu and Joshua Akey of the University of Washington; Alain Froment of France's Musée de L'Homme; Jean-Marie Bodo of Cameroon's Ministère de la Recherche Scientifique et de l'Innovation; Godfrey Lema and Thomas B. Nyambo of Tanzania's Muhimbili University College of Health Sciences; and Kun Zhang of the University of California at San Diego.

The researchers sequenced the genomes of five men from each of three hunter-gatherer groups: the Hadza and the Sandawe of Tanzania and the Western Pygmies of Cameroon. The three differ greatly from one another in appearance, in language, in the environments they occupy and in cultural practices, though the Hadza and the Sandawe live just 200 kilometers apart.

"We purposefully picked three of the most diverse hunter-gatherer groups," Tishkoff said, "because they have not been very well represented in other genome sequencing projects, which tend to focus on majority populations in Africa. This is a unique and important dataset."